The present invention relates to a data transmission system for transmitting data over telephone lines or similar analog lines and, more particularly, to a timing signal recovery circuit associated with a receiver of such a data transmission system.
Usually, the receiver of a data transmission system comprises a timing signal recovery circuit which generates a local timing signal necessary for recovering the transmitted data from a received signal. The conventional timing signal recovery circuit is composed of: a circuit to extract a timing signal from the received signal; a voltage controlled oscillator (VCO) to generate a local timing signal in a receiver; a phase difference detector for detecting the phase difference between the extracted timing signal and the generated local timing signal; and a loop filter for controlling the VCO in response to the detected phase difference. However, the timing signal recovery circuit of the kind has the disadvantage that if the loop filter generates initially a voltage larger than a voltage that can control the VCO, then the VCO may be driven into the uncontrolled condition. This causes in turn that an integrator contained in the loop filter integrate either excessive positive or negative voltage, with the result that the entire system becomes unstable.